The problem of obesity in children has spread beyond the West with Russia and China experiencing an alarming increase in the number of overweight youngsters, a new study reveals.

One in four children, aged between six and 18, in the United States are now overweight or clinically obese. But the same trend has been identified in developing nations where overeating and high-fat diets among affluent children are to blame.

In Russia, 16 per cent of youngsters are overweight or clinically fat and in China, seven per cent weigh too much.

The research, published in the International Journal of Epidemiology, comes after the World Health Organisation called for an urgent analysis of childhood obesity.

Dr Youfa Wang, an assistant professor of human nutrition at the University of Illinois in Chicago, used comparable data for more than 16,000 children across the three nations, which together account for a quarter of the world population.

"Although the problem of childhood obesity is much more serious in the US than Russia and China, we observes in those countries a remarkable increase, especially among younger children, those in urban areas and those in high socioeconomic groups," he concludes.

In Britain, a recent report by the National Audit Office suggested 13 per cent of children were too fat. A rapid rise in the problem meant that 2.6 per cent of girls and 1.7 per cent of boys were obese, while 13 per cent of girls and nine per cent of boys were overweight.

Dr Wang's study confirms that childhood obesity is a global problem, which tends to affect better-off families in developing countries and poorer people in the US.

In China, children with affluent parents in urban areas were the most likely to be obese because the families could afford to buy more food, particularly high-fat and high-density foods such as meat.

In Russia, children from the highest and lowest income groups were much more at risk than middle-income groups. The prevalence of obesity was also higher in rural areas.

In America, children from poorer families were most at risk because of bad diets.

Dr Wang also highlights the problem in Thailand, which he calls a transitional society. Obesity in schoolchildren here increased from 12 per cent to 16 per cent between 1991 and 1993.

In Brazil, the number of overweight and obese children had tripled over the past two decades to 14 per cent. Egypt had the same prevalence of fat children.